Anomalous Flattening of the Fast-Ion Profile during Alfvén-Eigenmode Activity

Neutral-beam injection into plasmas with negative central shear produces a rich spectrum of toroidicity-induced and reversed-shear Alfvén eigenmodes in the DIII-D tokamak. The first application of fast-ion ${\mathrm{D}}_{\alpha }$ (FIDA) spectroscopy to Alfvén-eigenmode physics shows that the central fast-ion profile is anomalously flat in the inner half of the discharge. Neutron and equilibrium measurements corroborate the FIDA data. The current density driven by fast ions is also strongly modified. Calculations based on the measured mode amplitudes do not explain the observed fast-ion transport.

Figure 1

(a) Cross power of radial and vertical ${\mathrm{CO}}_2$ interferometer channels showing the many RSAEs (upward-sweeping lines) and TAEs ($\sim$ horizontal lines) in the plasma. (b) Neutron rate and FIDA densities at $R=180$ and 195 cm vs time. The signals are normalized by the classical TRANSP neutron and beam-ion density predictions, respectively. The absolute calibrations of the neutron and FIDA data are adjusted so that the ratio is unity in the preceding 2.3 MW discharge at 2.0 s (when the Alfvén activity is undetectable).

Figure 2

Normalized neutron rate and $R=180\mathrm{cm}$ FIDA density versus approximate mode amplitude at various times in five successive discharges with increasing amounts of beam power. The dashed lines are linear fits to the data.

Figure 3

Fast-ion pressure profiles and FIDA density profiles versus $\rho$ at two different times that correspond to normalized neutron rates of 0.66 and 0.94. The dashed lines are the classical pressure profile predicted by TRANSP. The FIDA density profile is normalized to the MSE-EFIT $p_f$ profile at 1.2 s.

Figure 4

Measured evolution of $q_{\min }$ from MSE-based equilibrium reconstructions (line) and RSAE rational integer crossings (diamond) in discharges with (a) strong Alfvén activity (4.6 MW) and (b) weak Alfvén activity (2.3 MW). The TRANSP simulations assume either classical NBCD (solid line) or the NBCD from a centrally flattened fast-ion density profile (dashed line).

Figure 5

Change in the distribution function of fast ions with $E\ge 60\mathrm{keV}$ and $v_{\parallel }/v=0.4–0.7$ vs $\rho$; $\Delta F$ is normalized to the maximum value of the initial distribution function. The dashed lines are with collisions alone; the solid lines include collisions and 151 helical modes at 5 times the experimental amplitudes for the TAEs and RSAEs that are observed at 356 ms. The distribution function is sampled after 0.95–1.9 ms (green), 1.9–3.8 ms (turquoise), and 5.5–11 ms (purple). The (red) curve compares a TRANSP simulation with $D_B=5{\mathrm{m}}^2/\mathrm{s}$ inside $\rho =0.55$ and zero outside with a standard ($D_B=0$) simulation.